Functional devices comprising a charge transfer complex layer

ABSTRACT

Functional devices using charge transfer complexes formed as a layer on a substrate. In one embodiment of the invention, the device comprises a substrate, an electrode layer formed on one side of the substrate, at least one layer of a charge transfer complex capable of undergoing a variation in charge transferability by application of external energy, and another electrode layer formed on the complex layer. The charge transfer complex consists essentially of an electron donor and an electron acceptor, at least one of which is an organic compound having a long-chain alkyl substituent. In another embodiment, a charge transfer complex layer is provided on a substrate as polarized in one direction, in which the charge transfer complex does not necessarily contain such a substituted donor or acceptor compound as in the first embodiment but ordinary charge transfer complexes may be used. A pair of electrodes may be provided to sandwich the complex layer therebetween.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the use of charge transfer complexes and moreparticularly, to functional devices using such complexes in whichvariations of electric, optical or electromagnetic characteristics inthe device are utilized when external energy such as electric, opticalor electromagnetic energy, pressure and/or temperature is applied to thedevice. The functional device can be utilized by itself or incombination with other devices as switching devices, electric andoptical memories, optical devices utilizing optical waveguide, aphotovoltaic effect and photoconductivity, display devices and sensors.

2. Description of the Prior Art

As is well known in the art, organic charge transfer complexes have wideutility in various fields in which their specific characteristicsincluding electric conductivity or semiconductivity, electromotiveforce, dielectric properties, photoconductivity or the like areutilized. For instance, the complexes are utilized as polarized orionized. Alternatively, a neutral to ionic phase change of thesecomplexes are used to make various electric, electronic, optical orelectromagnetic devices.

It is known that a certain type of organic molecule crystal undergoes aphase transition of from a neutral crystal which may be a van der Waalscrystal to an ionized crystal wherein most constituent molecules aresubstantially ionized. For instance, it has been found that a singlecrystal of a charge transfer complex, for example, of tetrathiafulvalene(TTF) and chloranyl (CA) undergoes the phase transition at a relativelylow pressure of about 10 Kbar. In addition, when the temperature islowered down to not higher than 80° K. even at a normal pressure, thissingle crystal undergoes the neutral-ionic phase transition. Theneutral-ionic phase transition is considered to result from the balancebetween an energy loss of I_(D) -E_(A) wherein I_(D) is an ionizationpotential and E_(A) is an electron affinity and a gain of the Madelungenergy, aV wherein "a" is a Madelung constant and V is a coulomb energyof a D⁺ A⁻ pair wherein D is an electron donor and A is an electronacceptor in case where the electron donor and the electron acceptor areconverted from a neutral state (D°A°) into an ionized state (D⁺ A⁻). IfI_(D) -E_(A) >aV, the complex is neutral and if I_(D) -E_(A) <aV, thecomplex is ionized. In this connection, however, it has been recentlyfound that the neutral to ionic phase transition does not occur only bya simple change in the Madelung energy, but is greatly influenced by thecharge transfer interaction between the donor and acceptor molecules andalso by the electron-lattice interaction. For instance, theneutral-ionic phase transition of crystals of tetrathiafulvalene andchloranyl caused by application of temperature or pressure isaccompanied by the dimerization lattice strain. This means that asidefrom the energy balance depending upon the coulomb force, theelectron-lattice interaction which will cause Peierls deformation isimportant.

On the other hand, a Cu and tetracyanoquinodimethane (which may behereinafter referred to simply as TCNQ) complex is considered to undergothe neutral-ionic phase transition at room temperature under normalpressures. This complex may be formed by dissolving TCNQ in acetonitrileand subjecting the solution to reaction with Cu. When the complex issandwiched, as a layer, between Cu and Al electrodes and is applied witha voltage, a switching phenomenon is observed where the resistance iskept high by application of a voltage up to a certain voltage, andbecomes low when the applied voltage exceeds the level. Presumably, thisis because (Cu⁺ TCNQ⁻)_(n) is converted into a neutral phase whereCu_(x) ° and (TCNQ°)_(x) are formed. This ionic to neutral phase changemay be caused by application of light.

With the organic molecule crystals such as, for example, oftetrathiafulvalene and chloranyl, however, low temperatures orapplication of high voltages is necessary to cause the phase transitionto proceed.

On the other hand, crystals of (Cu⁺ TCNQ⁻)_(n) which undergo the phasetransition at normal temperatures and pressures are relatively unstableand are not reliable with respect to reproducibility. In addition, ithas been reported that the reason why the electric conductivityincreases after the transition to the neutral phase by the switchingphenomenon is due to an increase in amount of the Cu in the chargetransfer complex because of the evaporation of the TCNQ molecules causedby the Joule heat. Accordingly, the reproducibility of the switchingphenomenon is not necessarily reliable, thus presenting a practicalproblem in application of the complex as a function device.

On the other hand, charge transfer complexes consisting of electrondonors and acceptors are, in most cases, applied using their inherentelectric, optical and physical properties without use of such a specificphase transition as described above. For instance, many reports havebeen made with respect to conductivity, electromotive force, dielectricproperties and photoconductivity of charge transfer complexes. However,these complexes are usually polarized isotropically. When the directionof polarization is isotropic, any electric charge induced by thepolarization do not produce, or the photovoltaic force is offset andlessened. In addition, dielectric characteristics are also offset.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a functional device whichmakes use of a charge transfer complex of a specific type whereby theneutral-ionic or ionic-neutral phase transition can be caused to proceedstably and reproducibly.

It is another object of the invention to provide a functional device inwhich a charge transfer complex used consists of an electron donor andan electron acceptor, at least one of which has a long chain alkylsubstituent whereby the charge transfer complex undergoes a stableneutral-ionic phase transition at normal temperatures and normalpressures by application of external energy.

It is a further object of the invention to provide a functional devicewhich comprises a layer of a charge transfer complex which is polarizedin one direction, so that optical or electric or other physicalproperties of the layer are significantly improved upon application ofexternal energy.

In accordance with one embodiment of the invention, there is provided afunctional device which comprises a substrate, an electrode layer formedon one side of the substrate, at least one layer of a charge transfercomplex capable of undergoing a variation in charge transferability byapplication of external energy, and another electrode layer formed onthe complex layer. In the device, the charge transfer complex used ismade of an electron donor and an electron acceptor, one of which is anorganic compound having has a long-chain alkyl substituent. The alkylsubstituent has preferably from 10 to 22 carbon atoms. When externalenergy such as electric, optical or electromagnetic energy is applied tothe device, a neutral-ionic or ionic-neutral phase transition takesplace in the charge transfer complex layer. This phase transitionresults in the variation in electric, optical or electromagneticcharacteristics. Accordingly, the device may be used as an electric oroptical sensor. Alternatively, if the charge transfer complex usedundergoes a color change by application of an electric signal, thedevice may be used as a display device or memory.

In accordance with another embodiment of the invention, there is alsoprovided a functional device which comprises a substrate, and a layer ofa charge transfer complex which is formed on one side of the substrate.The charge transfer complex should be polarized in one direction. Inthis device, the charge transfer complex used may be any known organiccharge transfer complexes including not only those used in the firstembodiment, but also long-chain alkyl substituent-free charge transfercomplexes. This type of functional device may be applied as an opticalor display device or a memory. The charge transfer complex layer may besandwiched between a pair of electrodes. In this arrangement, anelectric charge induced by the polarization is produced on theelectrodes, resulting in an electromotive force. This force can beincreased by application of light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic section view of a functional device according toone embodiment of the invention;

FIG. 2 is a schematic view of a functional device according to anotherembodiment of the invention;

FIG. 3 is a schematic sectional view of a functional device according toa further embodiment of the invention;

FIG. 4 is a schematic view illustrating a functional device according toa still further embodiment of the invention;

FIG. 5 is a schematic view of a functional device according to anotherembodiment of the invention; and

FIG. 6 is a current-voltage characteristic of the functional deviceobtained in Example 1.

DETAILED DESCRIPTION AND EMBODIMENTS OF THE INVENTION

Broadly, the functional device according to the invention can beclassified into two classes. One class includes a layer of a chargetransfer complex which undergoes a neutral-ionic phase transition byapplication of external energy. This phase transition is effectivelyutilized as various types of devices including optic, electric,electro-magnetic devices or memories. The other class makes use ofpolarization of electron donor and acceptor which constitute a chargetransfer complex.

In accordance with the first class of the invention, there is provided afunctional device which includes a substrate, a first electrode, a layerof a charge transfer complex consisting of an electron donor and anelectron acceptor, and a second electrode formed on the substrate inthis order. At least one of the electron donor and acceptor should be anorganic compound which has a long-chain alkyl substituent. By this, theneutral-ionic phase transition proceeds in a reliable and reproduciblefashion upon application of a voltage across the charge transfer complexlayer or upon application of a laser beam or by heating the layer. Thereason why the substitution of a long-chain alkyl group leads to astable and reproducible phase transition is considered as follows: thelong-chain alkyl group enables the charge transfer complex to have afree steric space which permits the Peierls deformation to occur so asto cause a variation in the Madelung energy to be effectivelyreproduced. Moreover, the long-chain alkyl group is effective inimproving film-forming properties of the charge transfer complex.

Reference is now made to the the accompanying drawings and particularlyto FIG. 1. In FIG. 1, there is generally shown a functional device Dwhich includes a substrate 1. The substrate 1 has a transparentelectrode 2 such as an indium tin oxide film, a layer 3 of a chargetransfer complex of the type which will be described hereinafter, and acounter electrode 4 such as an indium tin oxide film. The electrodes 2and 4 have, respectively, silver paste connections 5 and 6 having leadsA and B. The substrate 1 may be made of glasses, plastic resins, metalsor the like. The electrodes 2, 4 are made, for example, of indium tinoxide, tin oxide, gold and the like. For optical purposes, the substrateand the electrodes should be optically transparent. The layer 3 is madeof a charge transfer complex of an electron donor and an electronacceptor. For ensuring a reliable and reproducible neutral-ionic phasetransition by application of external energy, at least one of the donorand acceptor should be an organic compound having a long-chain alkylsubstituents. Examples of the organic compounds free of the alkylsubstituent and capable of yielding charge transfer complexes incombination with an electron donor or acceptor includetetracyanoquinodimethane, p-benzoquinone, tetracyanobenzene and the likeas an acceptor, and p-hydroquinone, naphthalene anthracene,tetrathiafulvalene, basic dyes and the like as an electron donor.Examples of the basic dyes include Rhodamine, Crystal Violet, MethyleneBlue and the like. Other organic donors may be compounds of thefollowing formulae (I) and (II), and various derivatives thereof,capable of developing color by intramolecular ring closure or openingcaused by the charge transfer ##STR1##

The compound of the formula (II) is3,3-dimethyl-1,2-(p-dimethylaminostyryl)indolino[1,2-b]oxazoline.Various derivatives are described, for example, in U.S. Pat. No.4,147,862 and include3,3-dimethyl-5-methoxy-2-(p-chlorostyryl)indolino[1,2-b]oxazoline,3,3-dimethyl-5-chloro-2-(p-dimethylaminostyryl)indolino[1,2-b]oxazoline,3,3-dimethyl-2-(p-nitrostyryl)indolino[1,2b]oxazoline,3,3,5-trimethyl-2-(p-dimethylaminostyryl)indolino[1,2-b]oxazoline,3,3-dimethyl-5-methoxy-2-(p-chlorostyryl)indolino[1,2-b]oxazoline, andthe like.

Although not particularly indicated in this patent, mention is made ofuseful derivatives of the compound of the formula (I) including3,3-dimethyl1,2-(p-dimethylaminocinnamylidenevinyl)indolino[1,2-b]oxazoline and3,3-dimethyl-1,2-(p-dimethylamino-4-methylcinnamylidenevinyl)indolino[1,2-b]oxazoline.With respect to the compound of the formula (I), similar derivatives canbe prepared and used in the practice of the invention.

As a matter of course, metals such as Cu, Ag, a transition metal such asTi and the like may be used as a donor when the acceptor is asubstituted organic compound. On the other hand, a halogen ortetracyanoethylene may be used as an electron acceptor in which case thedonor should be an organic compound having a long alkyl substituent.

The above-mentioned organic donors and/or acceptors should have along-chain alkyl substituent. Typical examples of the substituted donorsand acceptors are shown below: ##STR2##

In the above formulae, each R or R' represents an alkyl group havingfrom 10 to 22 carbon atoms.

The above substituted compounds are know or can be prepared without anydifficulty. For instance, the substituted TCNQ (1) is prepared by aprocess described, for example, in U.S. Pat. No. 3,115,506 in which analkyl-substituted 1,4-cyclohexanedione is used as a starting material. Along-chain alkyl-substituted 1,4-cyclohexanedione can be prepared byhydrogenation of a long-chain alkyl-substituted hydroquinone. Thesubstituted indolinooxazoline (5) may be obtained by proceduresdescribed in the examples of the U.S. Pat. No. 4,147,862.

For ensuring the stable and reproducible phase transition, it isessential that the alkyl substituent be a long chain sufficient to allowa free space. For ease in preparation and economy, the long-chain alkylgroup has preferably from 10 to 22 carbon atoms including C₁₀ H₂₁, C₁₂H₂₅, C₁₈ H₃₇ or C₂₂ H₄₅. Preferable combinations of the donor andacceptor, at least one of which is substituted with a long-chain alkylgroup, include those of the alkyl-substituted TCNQ (1) and Cu or Ag, orthe indolinooxazoline compound of the formula (II) or derivativesthereof. Among the derivatives, 3,3-dimethyl1,2-(p-dimethylaminocinnamylidenevinyl)indolino[1,2-b]oxazoline and3,3-dimethyl-1,2-(p-dimethylamino-4-methylcinnamylidenevinyl)indolino[1,2-b]oxazolineare preferred because if a bias voltage is applied to a layer of thesederivatives, a clear color change occurs from colorless to blue. Thiscolor change reversibly proceeds: when the application of the biasvoltage is stopped, the layer returns to a colorless state. The compoundof the formula (II) likewise undergoes a clear colorless to red colorchange by application of a bias voltage. Accordingly, the functionaldevices using the above combinations can provide a display of goodquality. Further preferable combinations include those of thesubstituted oxazoline compound of the formula (5) and unsubstituted TCNQalthough other donative compounds indicated before may be used.

For the formation of the functional device D of FIG. 1, for instance, aglass substrate having a transparent indium tin oxide (ITO) electrode isprovided, on which a charge transfer complex is formed by deposition anda counter electrode is subsequently formed by ion plating or other knowntechniques. The charge transfer complex layer is usually formed bysubjecting either an organic donor or acceptor to vacuum deposition in apredetermined thickness and then the other to vacuum deposition,followed by heating the superposed layers up to 40° to 150° C. for atime sufficient to form a charge transfer complex. As will beappreciated from the above, the layer 3 may not necessarily be madeentirely of a charge transfer complex but may contain a sub-layerremaining as an electron donor or acceptor layer. Alternatively, thecharge transfer complex may be formed by dissolving electron donor andacceptor in an organic solvent and applying the resulting solution ontothe electrode-bearing substrate. Subsequently, the applied layer isheated to temperatures sufficient for the formation of an intendedcharge transfer complex.

The substrates may be in the form of a sheet, film or plate and theelectrodes are usually deposited in a thickness of from 300 to 3000angstroms. The layer 3 is usually formed in a thickness of from 50 to10,000 angstroms.

The other class functional devices of the invention are now describedwith reference to FIGS. 2 through 5.

FIG. 2 shows a functional device according to another embodiment of theinvention which is generally indicated by 10. The device 10 includes asubstrate 12, an electron acceptor layer 14 and an electron donor layer16 formed on the substrate 12. The electron acceptor layer 14 andelectron donor layer 16 may be reversed in the order. At an interface Ibetween the layers 14 and 16, polarization takes place due to the chargetransfer as schematically shown in the figure. In this figure, astrongly polarized charge transfer complex is shown in which dipolemoments are arranged in a direction as shown. In this condition, theelectric charge induced by the polarization is produced on the surfacesof the respective layers. The amount of the charge can be increased byapplication of light.

If a laser beam, hv, is passed along the direction of the arrow from theupper right side as viewed in FIG. 2, it is refracted at and passedalong the interface for the reason that the interface layer has anincreasing refractive index by the influence of the polarization.

Although the charge transfer between a donor and an acceptor may varydepending upon the height and shape of potential peaks of the donor andacceptor used, a complex whose charge transferability is poor will takean atomic arrangement of a substantially neutral molecule for both anacceptor and a donor, thus undergoing vibrations as molecules. As thecharge transfer between the donor and acceptor increases, they tend tobecome ionic in nature and vibrate as such. In either case, polarizationtakes place more or less.

In order to make a charge transfer complex polarized in a givendirection, donor and acceptor layers are superposed as shown in FIG. 2.Alternatively, a monomolecular layer of a charge transfer complex may beformed on a substrate by the Langmuir-Blodgett method as isschematically shown in FIG. 3. It should be noted that in FIGS. 2through 5, like reference numerals indicate like parts or members. Thedevice of FIG. 3 has a monomolecular layer of a charge transfer complexwhich is polarized. In order to enhance the functionality of the layer,it is preferred to form a plurality of the monomolecular layers in sucha way that the respective monolayers are polarized in the samedirection. This type of device will be particularly described in exampleappearing hereinafter.

FIG. 4 shows a functional device, generally indicated by 10, accordingto a further embodiment of the invention. The functional device of FIG.4 is similar to the device of FIG. 2 except that the acceptor layer 14and the donor layer 16 are sandwiched between a pair of electrodes 18and 20. In this embodiment, polarization takes place at the interfacebetween the acceptor and donor layers 14, 16. As in the case of FIG. 2,if a laser beam is applied to the device from the upper right as viewedin the figure, it passes along the interface for the reason describedbefore. Further, when a bias voltage is applied between the electrodes18, 20, the refractive index varies to cause optical modulation. FIG. 5shows a functional device using two monlayers between the electrodes 18,20.

The devices of FIGS. 2 to 5 may be applied as an optical sensor, anoptical waveguide or pressure or other sensors utilizing an inducedcharge as a variation in capacitance.

The substrates used in the devices of FIGS. 2 to 5 may be in the form ofa sheet, film or plate and may be made of glasses, plastic resins,metals and the like. If used, the electrodes may be made of thosedescribed with respect to FIG. 1.

The acceptors and donors useful in these embodiments may be not onlythose described with respect to the first class, but also ordinarilyemployed compounds. Such compounds are tetracyanoquinodimethane,p-benzoquinone, tetracyanoethylene, tetracyanobenzene and the like usedas an acceptor, and p-hydroquinone, naphthalene anthracene and metalssuch as copper, silver, a transition metal such as titanium and the likeas a donor. The basic dyes and the compounds of the formulae (I) and(II) indicated before may be likewise used as a donor. It should benoted that the charge transfer complexes used in this class may notnecessarily contain a long-chain alkyl group in at least one of a donorand an acceptor as in the first class embodiment.

In the second class, preferable combinations of donor and acceptorsinclude tetracyanoquinodimethane with or without substitution with along-chain alkyl group having from 10 to 22 carbon atoms and copper orsilver, or the compound of the formula (II) indicated before or aderivative of the compound. Preferable derivatives are those describedwith respect to the first class embodiment.

The present invention is more particularly described by way of examples.

EXAMPLE 1

This example describes a functional device of the type as shown in FIG.1.

A glass substrate having a transparent electrode of an indium tin oxidefilm on one side of the substrate was provided. Tetracyanoquinodimethane(TCNQ) having a C₁₈ H₃₇ substituent as shown in the following formula##STR3## was deposited on the electrode at 10⁻⁵ Torr., in a thickness of3000 angstroms. Subsequently, copper was subjected to vacuum depositionon the substituted TCNQ film under the same conditions as indicatedabove in a thickness of 50 angstroms. While keeping the vacuum, theglass substrate having the substituted TCNQ film and the Cu film washeated to 100° C. for 40 minutes including the heating time, therebyforming a (Cu⁺ -TCNQ⁻ (C₁₈)) complex layer in the films. An indiumtitanium oxide film was further formed on the layer by ion plating toobtain a device as shown in FIG. 1.

The device was subjected to visible light and infrared absorptionspectroscopic analyses. Formation of the charge transfer complex wasconfirmed from the visible light absorption spectra including a widecharge transfer band absorption at 600 to 900 nm and a gentle absorptionof the TCNQ ion radicals at a longer wavelength side of an absorption ofneutral TCNQ at 395 nm. In addition, the infrared absorption analysisrevealed the formation: the absorption of neutral TCNQ at 2228 cm⁻¹separated into absorptions at 2202 cm⁻¹ and 2162 cm⁻¹ and was thusshifted to a lower energy side. Moreover, an absorption at 2162 cm⁻¹became wider. Thus, the formation of the complex salt of the substitutedTCNQ and Cu was confirmed.

The device obtained above was allowed to stand in the air, whereupon anychanges were not recognized as determined through the visible lightspectroscopic analysis. Thus, the device was found to be stable.

With a device using unsubstituted TCNQ as an acceptor, it was foundthrough visible light spectroscopic analysis that part of the chargetransfer complex formed was returned into neutral Cu_(x) and (TCNQ)_(x)several hours after the fabrication of the device.

The device obtained in this example was subjected to measurement of acurrent-voltage characteristic. The characteristic is shown in FIG. 6.From the figure, it will be found that a negative resistancecharacteristic of the N type is obtained. This characteristic issymmetric irrespective of the polarity of the applied voltage as isparticularly shown in FIG. 6. When the bias voltage is turned off, thedevice returns to an original state or a low resistance level.

The negative resistance characteristic is considered due to theionic-neutral phase transition occuring as follows:

    (Cu.sup.+ ·TCNQ(C.sub.18).sup.·).sub.n ⃡Cu.sub.x.sup.o +TCNQ(C.sub.18).sub.x.sup.o +(Cu.sup.+ ·TCNQ(C.sub.18  ).sub.n-x

The substituted TCNQ is unlikely to evaporate by the Joule heat as inprior art acceptors or donors, resulting in good reversability of thephase change.

It was confirmed that the above phase transition took place byapplication of light such as a semiconductor laser beam, whereupon thechange transfer band in the visible light spectra disappeared, enablingone to make optical writing.

In addition, it was also found that the phase transition occurred byreducing a pressure or increasing a temperature so as to allow a widerdistance between Cu⁺ and the substituted TCNQ⁻.

Because the complex salt suffers a sharp variation in magneticsusceptibility between the ionic and neutral phases, the phasetransition may be caused by application of a magnetic field.

In the above example, Cu was used as an electron donor, Ag or atransition metal such as Ti could be likewise used. Moreover, the C₁₈H₃₇ alkyl substituent was used as a long alkyl group, but use of otheralkyl substituents such as C₁₀ H₂₁, C₁₂ H₂₅ and C₂₂ H₄₅ leads to similarresults.

The charge transfer complex layer was formed by vacuum deposition, butmay be formed by application of a solution of a charge transfer complexonto the substrate or by the Langmuir-Blodgett method.

EXAMPLE 2

This example illustrates a functional device as shown in FIG. 2, inwhich a charge transfer complex is polarized in a given direction.

A glass substrate was provided, onto which tetracyanoquinodimethane wasvacuum deposited in a thickness of 1000 angstroms under conditions of10⁻⁵ Torr., to form an acceptor layer, followed by vacuum deposition ofa dye of the formula (1) indicated before in a thickness of 1000angstroms to form a donor layer. When the resultant device was allowedto stand at normal temperatures, charge transfer took place at theinterface between the acceptor and donor layers as shown in FIG. 2. TheC--O bond of the dye is broken as shown in the following formula (III)at the occurrence of the charge transfer, thereby producing acolor-developing species. ##STR4## When an argon laser beam of 530 nmwas applied to the superposed layers, it was refracted at the interfaceand passed therealong. This is considered due to a high refractive indexat the charge transferred region.

The charge induced by the polarization can be measured at the surface ofthe donor layer. When plural acceptor and donor layers are superposed sothat the resulting charge transfer complex is polarized in the samedirection at the interfaces, an amount of the charge increases.

EXAMPLE 3

Substituted tetracyanoquinodimethane of the following formula (IV)##STR5## in which R represents a C₁₈ H₃₇ group, and a dye of the formula(II) indicated before were dissolved in chloroform at a molar ratio of1:1. When the resultant solution was allowed to stand for 30 to 60minutes, a complex was gradually formed while developing a red color.This solution was developed over a water surface to form a monomolecularlayer, followed by deposition on a glass substrate by a horizontaldeposition technique. This is schematically shown in FIG. 3, in whichthe donor 16 is a dye molecule and the acceptor 14 is the substitutedTCNQ molecule. The color development is attributed to the breakage ofthe C--O bond of the dye as shown in the following formula (V) ##STR6##

The absorption spectrum analysis reveals that an absorption maximumappears at 570 nm but the absorption of the substituted TCNQ is rarelyobserved. From this, it is considered that the molecules of thesubstituted TCNQ are arranged vertically with respect to the substrateand the dye molecules are arranged parallel as shown in FIG. 3.

The charge induced by the polarization was observed on the surface ofthe layer. In this case, this charge increases when a plurality of themonomolecular layers are superposed as in Example 2.

In Examples 2 and 3, the acceptor and donor layers are superposed on thesubstrate in this order but may be reversed.

EXAMPLE 4

The general procedure of Example 2 was repeated except that indium tinoxide electrodes were formed prior to and after the formation of theacceptor and donor layers, each in a thickness of 200 angstroms, therebyobtaining a functional device as schematically shown in FIG. 4. In thiscase, the induced charge could be measured between the electrodes as anelectromotive force. For increasing the electromotive force, a pluralityof donor and acceptor layers should be formed to have the same directionof polarization at the respective interfaces.

EXAMPLE 5

The general procedure of Example 3 was repeated except that indium tinoxide electrodes were formed prior to and after the formation of themonolayer, and two monolayers were formed as schematically shown in FIG.5, thereby obtaining a functional device having the electrodes.

In the devices obtained in Example 4 and 5, the charge transfer complexis arranged as a layer having a given direction of polarization and issandwiched between electrodes. The charge induced by the polarizationcan be measured, and increases by application of light. Accordingly,these devices can be used as an optical sensor. Alternatively, therefractive index at the polarized portion increases, so that the devicescan be used as an optical waveguide. These functions can be modulated byapplication of a bias voltage between the electrodes. The devices of theinvention have thus wide utility in various fields.

What is claimed is:
 1. A functional device which comprises a substrate,an electrode layer formed on one side of the substrate, at least onelayer of a charge transfer complex formed on the electrode layer andcapable of undergoing a neutral-ionic phase transition and a variationin charge transferability by application of external energy, and anotherelectrode layer formed on the complex layer, said charge transfercomplex consisting essentially of an electron donor and an electronacceptor, at least one of which is an organic compound with a long-chainalkyl substituent having from 10 to 22 carbon atoms.
 2. A functionaldevice according to claim 1, wherein said electron acceptor is a memberselected from the group consisting of tetracyanoquinodimethane,p-benzoquinone, and tetracyanobenzene, each substituted with an alkylgroup having from 10 to 22 carbon atoms.
 3. A functional deviceaccording to claim 1, wherein said electron donor is Cu or Ag and saidelectron acceptor is tetracyanoquinodimethane substituted with an alkylgroup having from 10 to 22 carbon atoms.
 4. A functional deviceaccording to claim 2, wherein said alkyl group has 18 carbon atoms.
 5. Afunctional device according to claim 1, wherein said electron donor istetrathiafulvalene and said electron acceptor istetracyanoquinodimethane substituted with an alkyl group having from 10to 22 carbon atoms.
 6. A functional device according to claim 1, whereinsaid electron donor is a dye of the following formula (I) or (II) or aderivative thereof ##STR7## and said electron acceptor istetracyanoquinodimethane substituted with an alkyl group having from 10to 22 carbon atoms.
 7. A functional device according to claim 6, whereinsaid electron donor is the dye of the formula (II).
 8. A functionaldevice according to claim 6, wherein said electron donor is3,3-dimethyl-1,2-(p-dimethylaminocinnamylidenevinyl)indolino[1,2-b]oxazoline.9. A functional device according to claim 6, wherein said electron donoris3,3-dimethyl-1,2-(p-dimethylamino-4-methyl-cinnamylidenevinyl)indolino[1,2-b]oxazoline.10. A functional device according to claim 1, wherein said electrondonor is a dye of the following formula ##STR8## in which R representsan alkyl group having from 10 to 22 carbon atoms, and said electronacceptor is unsubstituted tetracyanoquinodimethane.
 11. A functionaldevice according to claim 1, wherein said charge transfer complexconsists essentially of copper and tetracyanoquinodimethane substitutedwith an alkyl group having from 10 to 22 carbon atoms.
 12. A functionaldevice which comprises a substrate, at least one layer of a chargetransfer complex which is formed on one side of the substrate, and apair of electrodes sandwiching said at least one layer therebetween,said charge transfer complex being polarized in one direction, said atleast one layer is made of an acceptor sub-layer and a donor sub-layersuperposed one on another, and said charge transfer complex is formedand polarized at the interface between said acceptor sub-layer and saiddonor sub-layer.